A laser designator is a laser light source which is used to designate a military target. Laser designators provide targeting for laser guided bombs, missiles, and precision artillery munitions. The designator is used to apply laser light to the target, causing the light to bounce off the target into the sky, where it is detected by the seeker on a laser guided munition, which steers itself towards the source of the reflected light.
Typically, when a target is marked by a designator, the beam does not shine continuously. Instead, a series of pulses of laser-light are fired, and the pulsing rate is used as an identifying code. This allows a plurality of targets to be simultaneously marked with different pulsing rates, so that each of a corresponding plurality of munitions can be programmed to recognize and be guided only by a laser designation having a specific pulsing rate.
A simplified diagram of a typical laser designator of the prior art is presented in FIG. 1. A laser gain medium such as neodymium doped yttrium aluminum garnet (ND:YAG) 100 is located in the center of an optical resonator and is pumped from the side by pumping diodes 102. The optical resonator is terminated by a horizontal Porro prism 104 at one end and by a vertical Porro prism 106 at the other end. A passive or active Q-switch 108 located within the resonator causes the laser light to be emitted in pulses, and a quarter wave plate 110 in combination with a polarized reflector 112 and a mirror 114 provide output 116 from the resonator.
The use of Porro prisms 104, 106 instead of end mirrors to terminate the ends of the optical resonator path eliminates the need for critically accurate alignment of the resonator components. In addition, use of the Porro prisms 104, 106 significantly reduces the sensitivity of the resonator to thermal effects, such as thermal expansion of the mounting platform (not shown, typically aluminum), thermal lensing within the laser, thermal drift of the resonator components 110, 112, and thermal drift of the pulsing rate (in the case of a passive Q-switch 108).
Unfortunately, currently deployed laser designating systems such as the design illustrated in FIG. 1 tend to be heavy and bulky, and require assembly in the field, thereby reducing their effectiveness, especially for dismounted observers. The bulk and weight of the units also prevents deployment of laser designating systems as one-man portable systems, for example by integrating the designator into a self-contained sight that also provides precision targeting data.
Much of the weight and bulk of the current designs arises from the use of a side-pumped laser gain element 100. Attempts have been made to design a more compact and lighter laser designator using an end-pumped gain element 100. An example is illustrated in FIG. 2, where a Nd:YAG gain medium 100 is pumped from one end by pumping diodes 102. In this arrangement, a highly reflective mirror 204 deposited directly on the gain element itself 100 terminates the optical resonator path at one end, and a single partially transmitting mirror 202 terminates the resonator path at the other end. This approach minimizes the number of components, and reduces bulk and weight by using an end-pumped gain element 100.
However, since the highly reflective mirror 204 is deposited on the gain element 100 at one end of the resonator path, it is not possible to use a pair of Porro prisms as the terminators of both ends of the resonator path. As a result, the alignment of the resonator components 100, 108, 202 is highly critical.
In the design illustrated in FIG. 2, “monoblock” construction is used to directly and rigidly bond and align all of the resonator elements on a common, temperature compensated, optical quality support rail 200. However, this approach significantly increases the cost of materials and of assembly. In addition, even if the internal temperature of the designator is regulated, the use of an optical-quality rail 200 does not compensate for thermal lensing of the gain medium 100 and the resulting mirror misalignments due to variations in average output power due to differing pulse rate codes.
In addition, the length of the gain medium 100 must be sufficient to provide both the required target designation brightness and the laser pulse width, which places a limit on the minimum length of the designator.
What is needed, therefore, is a high beam quality laser designator design that is insensitive to temperature and to alignment, compact and lightweight, and low in manufacturing cost.